1. Technical Field to which the Invention Belongs
The present invention relates to a piezoelectric element, an inkjet head, a method for fabricating the piezoelectric element and the inkjet head, and an inkjet recording apparatus.
2. Prior Art
A piezoelectric material converts mechanical energy into electric energy or electric energy into mechanical energy. A typical example of the piezoelectric material is lead zirconate titanate (Pb(Zr,Ti)O3) (hereinafter referred to as “PZT”), that is, an oxide with a perovskite type crystal structure. In using the PZT with a tetragonal perovskite crystal structure in particular, a maximum piezoelectric displacement can be obtained along the <001> axis direction (i.e., the c-axis direction). However, most of the piezoelectric materials are polycrystals of aggregates of grains, and the crystal axes of the respective grains extend in various different directions. Accordingly, the directions of the spontaneous polarizations Ps are various.
Now, in accordance with recent downsizing of electronic equipment, there have been strong demands also for the downsizing of a piezoelectric element. In order to meet the demands, a piezoelectric element in the form of a thin film, which has a remarkably smaller volume than a conventionally frequently used sintered body, has become used in more and more cases. Therefore, various developments and studies have been earnestly made for thinning the piezoelectric element.
For example, since the spontaneous polarization Ps of the PZT extends along the <001> axis direction, in order to achieve a PZT thin film with high piezoelectric properties (piezoelectric displacement properties), it is necessary to make the <001> axis of the crystal constituting a PZT thin film extend along the vertical direction to one face corresponding to one end thereof along the thickness direction of a substrate. For this purpose, in conventional technique, on a monocrystal substrate of magnesium oxide (MgO) with a rock-salt crystal structure having the (001) plane on a top face thereof, a PZT thin film with good crystallinity and with the <001> axis oriented vertically to one face corresponding to one end thereof is directly formed at a temperature of 600–700° C. by sputtering using PZT as a target (for example, see Japanese Laid-Open Publication No. 10-209517). This method is characterized by using the substrate of MgO monocrystal, and owing to this substrate, a piezoelectric thin film with high piezoelectric properties and preferred orientation along the crystal direction can be achieved.
The MgO monocrystal is, however, a very expensive material, and therefore, this method is not preferred from the viewpoint of cost when it is applied to mass production of industrial products of piezoelectric elements and the like using the piezoelectric thin film.
In contrast, as a method for forming the (001) plane or (100) plane crystal orientation film of a perovskite material such as PZT on an inexpensive substrate of silicon or the like, there have been known methods. The following method, i.e., a method disclosed in Japanese Patent Publication No. 3021930 is one of the examples, shows that by applying a PTZ precursor solution containing PZT or lanthanum onto a Pt electrode oriented along the (111) plane, thermally decomposing the precursor at a temperature of 450–550° C. and then thermally treating the precursor at 550–800° C. to crystallize the precursor (zol-gel method), a (100) plane crystal orientation film of PZT can be generated.
However, when a piezoelectric element is mass produced by the zol-gel method, crystallization is caused in the process step of removing an organism and the process step of heating an amorphous piezoelectric precursor film at a high temperature to crystallize the film, so that cracks and peelings between a lower electrode and a piezoelectric thin film occur in many cases.
In contrast, as a method for forming a crystal orientation film on an inexpensive substrate, a method for synthesizing the crystal orientation film not requiring a zol-gel method and employing, for example, sputtering is known (see, e.g., Japanese Laid-Open Publication No. 2004-79991). Now, procedures for forming a crystal orientation film by this method will be described. First, an electrode thin film made of a noble metal alloy of Pt or Ir including Co, Ni, Mn, Fe, or Cu is deposited on a substrate as an underlying electrode by sputtering. Next, PZT is deposited over the electrode thin film by sputtering. Thus, a PZT thin film with the (001) crystal orientation can be obtained.
The piezoelectric thin film formed in the aforementioned manner exhibits a large piezoelectric constant, and large piezoelectric displacement is caused even when the applied voltage is low. For this reason, such a piezoelectric thin film is expected to be used as an actuator in a variety of fields. Also, when a high voltage is applied to the piezoelectric thin film, even larger piezoelectric displacement can be caused.
However, when a voltage is applied to an actuator including a PZT thin film formed by sputtering for a long time in a high-temperature and high humidity atmosphere (at a temperature of 50° C. and a humidity of 50%), the amount of displacement is reduced and the electrode thin film is turned black, resulting in deterioration of the actuator. It can be considered that this is caused by a reaction of excessive Pb with water at an interface between the electrode thin film and the PZT thin film.
The present invention has been devised in view of the above-described point, and it is therefore an object of the present invention to provide, at low cost, a piezoelectric element having excellent piezoelectric properties and high moisture resistance, an inkjet head including the piezoelectric element, and an inkjet recording apparatus including the same.